Capacitors are components of common use in most of electronic circuits Referring in particular to an electronic device integrated in a chip of semiconductor material, different techniques are known to make the capacitors on the same chip in which there are made other components of the electronic device, both passive ones (such as resistors) and active ones (such as transistors).
For example, (integrated) capacitors of Metal-Oxide-Metal (or MOM) type are formed by two metal plates enclosing a silicon oxide layer; MOM capacitors are currently the preferred choice in many kinds of electronic devices because of their implementation simplicity.
On the contrary, capacitors of Metal-Insulator-Metal (or MIM) type are formed by two metal plates enclosing an insulating layer (for example, silicon nitride). The silicon nitride has a high dielectric constant, so that the MIM capacitors have a specific capacity (per unit area of their plates) that is greater than that of the MOM capacitors; this allows making more compact capacitors that take up a smaller area of the chip (for the same overall capacity). Moreover, the MIM capacitors do not exclude the possibility of integrating other components under them (with a further saving of area of the chip).
However, the MIM capacitors suffer from some drawbacks that may limit their use (in spite of their better electrical characteristics with respect to the MOM capacitors).
In particular, when MIM capacitors are integrated with power components, the bottom plate may have a relatively large thickness; in fact, the bottom plate is made through a dedicated portion of a buried metal layer of the electronic device, whose thickness depends on the requirements of its power components. For example, in case of bipolar-CMOS-DMOS (BCD) technology—wherein BJTs for precision analog applications, CMOSs for digital applications, and DMOSs for power applications are integrated in the same chip—such metal layer has a typical thickness of approximately 0.7-1.0 μm (in order to reduce the ON resistance of the power components and to also reduce the electro-migration phenomena).
However, the relatively large thickness of the bottom plate may cause the formation of hillocks because of its thermo-plastic deformation due to the high temperatures to which the bottom plate is subject during the operations following its construction; moreover, this involves a thick grain structure of the bottom plate, which causes kinks and superficial irregularities. As a consequence, electric fields having high intensity may form between the plates, thereby reducing a voltage breakdown of the capacitor.
On the contrary, the top plate may have a thickness being less than that of the bottom plate; in fact, the top plate may be made within an insulating layer protecting the metal layer in which the bottom plate is made, so that the thickness of the top plate is significantly lower than the thickness of the insulating layer; in the same above-described BCD technology wherein the insulating layer has a typical thickness of approximately 0.7-1.0 μm, the thickness of the top plate may be of the order of approximately 0.1-0.3 μm.
However, the relatively large difference between the thickness of the bottom plate and the thickness of the top plate (for example, in a ratio of about 4-5) may cause imbalances that involve mechanical stresses on the capacitor. Moreover, the construction of a contact terminal (or simply contact) of the top plate may be critical. In fact, for this purpose it may be necessary to open a corresponding hole through the overlying insulating layer by means of an etching process; such insulating layer, being thick enough in correspondence of the metal layer used by the bottom plate and by the other components of the electronic device, may be thinner in correspondence to the top plate placed above such metal layer (for example, approximately 0.4-0.8 μm); therefore, there exists a risk of reaching the insulating layer that separates the two plates, thereby causing the destruction of the capacitor. In order to avoid such problem, one may use detection systems to detect a stop-point of the etching process; however, this may require the use of very sophisticated and expensive machinery, with a consequent increase in the production cost of the capacitor and hence of the whole electronic device.